Method of coating magnetic sheet material



United States Patent METHOD OF COATING MAGNETIC SHEET MATERIAL Warren M. Trigg, Pittsburgh, and Byron V. McBride,

Irwin, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application August 26, 1954 Serial No. 452,472

4 Claims. (Cl. 29155.61)

The present invention relates to magnetic sheet material and has particular reference to the provision of adherent electrically insulating coatings for application to magnetic sheet materials.

It is a common practice in the electrical industry when manufacturing cores for transformers, magnetic amplifiers, and the like, to provide the magnetic sheet material employed therein with a coating of magnesium oxide. One known method for applying such a coating consists of preparing a slurry in which water and magnesium oxide are the sole ingredients, applying the slurry to magnetic sheet material, and then drying the applied slurry to remove substantially all the water. The coating of magnesium oxide thus deposited is not completely satisfactory because it does not adhere tenaciously to the magnetic sheet material and tends to fall oif readily during the usual operations of building magnetic cores therefrom. As a result, after the coated sheet material has undergone core punching or core winding, annealing, and other necessary handling operations, only a relatively small proportion of the magnesium oxide coating is found between the windings or laminations of the core. Uncoated areas of the magnetic sheet will short circuit on contact and have high alternating-current losses.

The object of the present invention is to provide an adherent electrically insulating coating composition adapted to be applied to magnetic sheet material prior to winding or other fabricating operations of the magnetic material into cores or the like.

Another object of the invention is to provide a process for applying an adherent electrically insulating coating to magnetic sheet material.

A still further object of the invention is to provide magnetic sheet material with an adherent electrically insulating coating which is highly resistant to flaking and powdering on handling.

Other and further objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

It now has been discovered that the above and other and further objects are obtained when magnesium oxide is applied to magnetic sheet material from a coating composition containing a thermosetting aminotriazine-aldehyde resin, particularly melamine-formaldehyde resins. Coating compositions containing such resins possess unexpected characteristics not possessed by other known coating compositions whereby unusually effective, tenaciously adhereing electrically insulating refractory film coatings are provided on magnetic sheet material.

Broadly, the coating compositions of this invention comprise an aqueous composition consisting essentially of from 5% to 15% by weight of at least one partially reacted aminotriazine-aldehyde resin which is soluble in water-alcohol mixtures, from 5% to 15% by weight of magnesium hydroxide, from 0.1% to 1% by weight of bentonite, from 30% to 50% by weight of at least one low molecular weight aliphatic monohydric alcohol, and from 30% to 50% by weight of water.

In producing magnetic sheet material with an adherent electrically insulating coating in accordance with this invention, the liquid coating composition is first applied to the sheet material in any convenient manner. To illustrate, the coating may be applied simply by dipping the sheet material in the coating composition; or the composition may be rolled onto the sheet material using rubber or felt rollers; or the composition, after being applied as a heavy, non-uniform coating by dripping or the like, may be reduced to a thin film coating by passing the sheet material under a knife blade; or electrophoretic deposition techniques may be used. The applied coating of the liquid composition is heated to a temperature high enough to drive oil the water and the alcohol and effect curing of the resin. Temperatures within the range of from C. to about 150 C. are satisfactory to achieve these results. The coating thus deposited on the sheet material adheres thereto tenaciously. The coated sheet will withstand the relative rough handling frequently encountered during shipping ofthe sheet material to fabricating plants or to core winding stations. Little, if any, of the coating flakes oif during such handling operations or during subsequent punching, core winding and fabricating operations or during the final annealing operation. The wound cores are annealed at temperatures within the range of from about 900 C. to 1300 C. in an atmos phere of a reducing gas to remove strains set up by punching and other fabricating operations. At such high temperatures, the aminotriazine-aldehyde resins decompose and the decomposition products are carried 01f by the reducing gas leaving a tenaciously adhering solid coating on the sheet material consisting essentially of magnesium oxide and bentonite.

The resin employed in the coating composition of this invention is produced by condensing formaldehyde with an aminotriazine or condensed aminotriazone. Among these amino compounds, melamine and its derivatives such as 2,4,6-triethyl-and-tiiphenyl-triamino-1,3,5- triazines, 2,4,6-trihydrazino-1,3,5-triazine and the corresponding condensed triazines such as melam and melem are preferred. Among the other triazine compounds which may be used, the following are included: triazines containing one or two amino groups such as ammeline, ammelide, formoguanamine, 2-amino-1,3,5-triazine and their substitution products as well as nuclear substituted aminotriazines such as 2-chloro-4,6-diarnino-l,3,5-triazine, 2-phenyl-4-amino-6-hydroxy-1,3,5-triazine, 6-methyl-2,4- diamino-l,3,5-triazine. Obviously, commercial mixtures of the various tria'zines or mixtures thereof with other amino compounds may be used if desirable. One example of such commercial mixtures is the product obtained by heating dicyandiamide sufiiciently to obtain a significant amount of melamine, together with other reactive amino bodies.

The aminotriazines mentioned above are preferably partially condensed with formaldehyde but any suitable aldehyde of the aliphatic, aromatic or heterocyclic series may be used in the reaction such as acetaldehyde, propionaldehyde, butyraldehyde, hexaldehyde, heptaldehyde, crotonaldehyde, allylaldehyde, benzaldehyde, cinnamylaldehyde, furfural, etc.

The condensation products may be produced by any suitable process and with any desired combining ratio of aldehyde to aminotriazine from 1:1 up to 6:1, or even higher.

A particularly suitable resin is the melamine reaction product of 1,3,5-triamino-2,4,6-triazine and formaldehyde. This reaction product may be prepared conveniently by reacting the components for a brief period of time at about 80 C. to C. in the presence of an alkaline catalyst such, for example, as lime, barium hydroxide, ammonia, ethylenediamine, and propylenediamine. The amount of catalyst is of the order of onehalf percent. The resinous reaction product should be soluble in 25% aqueous ethanol.

It is preferred to employsubs-tantially completely hydrated magnesium hydroxide in preparing the aqueous coating compositions of this invention. Magnesium oxide may be employed in the aqueous coating composition although additional Water must be present to be taken up as waterof hydration since magnesium oxide will tend to take up water on standing thereby changing the predetermined viscosity of the aqueous composition. From about 5% to 50% by weight of the magnesium hydroxide employed in the aqueous coating composition may be replaced by one or more finely divided inert refractory insulating solids selected from the group consisting of metal oxides, metal silicates and metal phosphates. Examples of such refractory solids are aluminum oxide, zirconium oxide, silica, iron oxides, calcium silicate, aluminum silicate, calcium phosphate, magnesium silicate and magnesium phosphate.

The bentonite employed inthe aqueous composition is a naturally occurring colloidal crystalline inorganic aluminum silicate. A variety which has been found to be particularly suitable 'for'use in forming the coating compositions of the present invention is that known as montmorillonite.

The alcohol is an essential ingredient in the compositions of this invention. The partially reacting aminotriazine-aldehyde resin is not completely soluble in water. Alcohol functions somewhat in the manner of a common solvent in the aqueous compositions. Alcohols which have been found to be particularly suitable for use in the coating compositions are those aliphatic monohydric alcohols having 1 to 4 carbon atoms including methyl, ethyl, propyl, and isopropyl alcohol.

In order to indicate even more fully the advantages and capabilities of the present invention, the following examples are set forth. These examples are presented for illustrative purposes only and they are not intended to limit the scope of the invention in any manner. The parts given are by weight unless otherwise indicated.

Example I A mixture is prepared by introducing about 4 parts of bentonite into about 1500 parts of ethyl alcohol (90%) and about 1500 parts of water. The mixture is stirred for about to 20 minutes whereupon about 350 parts of a partially reacted melamine-formaldehyde resin is added over a period of about 10 minutes with stirring. Thereafter, about 350 parts of magnesium hydroxide are added slowly and the resulting mixture is stirred for about minutes. The mixture is then passed through a mesh screen to remove any large particles and then through a paint mill whose stones are not more than 0.001 inch apart to yield a smooth uniform coating composition. 7

Coating compositions which are equally satisfactory to that described in Example I may be prepared by replacing from 5% to 50% by weight of the magnesium hydroxide with aluminum finely divided inert refractory solids. It is preferable that these latter solids be of a fineness to pass through a sieve having 200 meshes per lineal inch or finer.

Example II 10 seconds whereby water and alcohol are driven off and the resin is substantially completely thermoset.

The dry coating thus developed on the strips of magnetic material is very adherent whereby the strips may be shipped with little, if any, of the coating composition coming loose.

Example HI A magnetic core is prepared by passing strips of magnetic material through an aqueous coating composition, prepared as described in Example I, which has the following composition:

Parts Melamine-formaldehyde resin (aqueous-alcohol soluble) 470 Magnesium hydroxide v 210 Bentonite 4 Isopropyl alcohol 1500 Water 1500 The strips with the applied liquid coatings then are passed through a furnace provided With suitable heating elements maintained at a temperature sufficiently high to effect vaporization of the water and alcohol and to complete the curing of the resin to a thermoset material. The strips having dry, tenaciously adhering coatings, then are introduced onto a core winding machine where they are Wound into an assemblage of superimposed laminations in the form of a wound magnetic core. The wound core then is placed in an annealing furnace and heated to a temperature of about 1050 C. to 1230 C. in the presence of a moving stream of hydrogen, whereby all traces of the alcohol, water, and melamine-formaldehyde resin and decomposition products thereof are removed. The

' annealed magnetic core thus produced comprises a pluoxide, silica flour, or other e A composition composed of 210 parts of a partially being exposed to this temperature for approximately 5 to rality of superimposed sheets of magnetic material having a thin electrically insulating coating therebetween consisting essentially of magnesium oxide and bentonite. The cores built up in this manner have a space factor of about 99% As indicated hereinabove, the coating compositions of this invention may be applied to magnetic sheet material by electrophoretic deposition. The following example describes such a process.

Example IV A coating composition is prepared according to the method describedv in Example I utilizing the following ingredien The composition is introduced into a cell having a stainless steel blade as an anode. A one-quarter inch wide strip of a magnetic material namely an alloy (nickel 5 0%, iron 50%) of a thickness of 0.002 inch is passed through the cell, the strip being made the cathode. The pH of the coating composition is maintained at about 9.5 to 10.5. An electrical current of volts direct current is applied to the strip and the anode, the two being separated to pass a current of 55 amperes per square foot of the strip in the bath. The strip is passed through the bath at a speed of approximately 30 feet per minute, about 1% feet of the strip being immersed in the bath. A coating of a thickness of about 0.001 inch is deposited on the strip, which on drying at a temperature of C. results in a shrinkage of the coating to about 00001 inch.

The coated strip thus prepared may be wound into magnetic cores and annealed at temperatures of about 900 C. to 1300 C. to provide a tenaciously adhering coating of magnesium oxide andbentonite.

The thickness of the layer of the liquor coating composition applied to the magnetic sheet material preferably is within the range of about 25% to 150% of the thickness of the sheet. For extremely thin gauge magnetic sheet material, namely from 0.004 inch to 0.0005 inch thick silicon-iron alloys, nickel-iron alloys, cobalt-iron alloys, and the like, the applied coating preferably is from 50% to 150% of the thickness of this sheet. For heavier magnetic steel of the order of 14 to 22 mils, the coatings may be from 5 to 15 mils in thickness. After heat treatment during which the water and alcohol are driven off and the resin cured, the coatings shrink considerably, and constitute only about 5% to 20% of the thickness of the applied liquid composition.

The coated sheet magnetic material may be punched after the resin curing heat treatment to produce rotor and stator laminations, transformer punchings, and other magnetic laminations. Furthermore, the coated magnetic sheet material may be cut into strips and wound into cores with little, if any, of the coating coming loose therefrom.

When employed in cores, rotors, and stators the coated magnetic sheet materials produced in accordance with this invention may be subjected to varnish impregnation and other conventional insulating treatments without causig undesirable changes in the varnishes or other insulating compounds.

The coating compositions of this invention are extremely adherent to magnetic sheet materials and have been found to be impervious to oils, moisture, and commonly encountered solvents. The electrical resistance of the coatings is outstanding, particularly when their extreme thinness is considered.

While the present invention has been disclosed with reference to particular examples and embodiments thereof, it will be understood, of course, that numerous changes, substitutions and modifications may be made therein without departing from its true scope.

We claim as our invention:

1. In the method of forming magnetic cores, the steps comprising applying to magnetic sheet material a layer of an aqueous composition consisting essentially of from 5% to 15% by weight of at least one aminotriazine-aldehyde resin, from 5% to 15 by weight of magnesium hydroxide, from 0.1% to 1% by weight of bentonite, from 30% to 50% by weight of at least one low molecular weight aliphatic monohydric alcohol, and from 30% to 50% by weight of water, heating the applied layer to a temperature of about 150 C. to drive off the water and alcohol and cure the resin, superimposing a plurality of sheets of said coated magnetic material, and heating the superimposed sheets at a temperature above 900 C. in a reducing atmosphere to drive ofi the resin whereby an electrical insulating coating of magnesium oxide and bentonite'is left between the sheets.

2. The method of claim 1 wherein from 5% to of the weight of the magnesium hydroxide is replaced by at least one finely divided inert refractory solid selected from the group consisting of metal oxides, metal silicates, and metal phosphates.

3. In the method of forming magnetic cores, the steps comprising applying to magnetic sheet material a layer of a thickness of from about 25 to about of the thickness of the magnetic sheet material of an aqueous composition consisting essentially of from 5% to 15% by weight of at least one aminotriazine-aldehyde resin, from 5% to 15% by weight of magnesium hydroxide, from 0.1% to 1% by weight of bentonite, from 30% to 50% by weight of at least one low molecular weight aliphatic monohydric alcohol, and from 30% to 50% by weight of Water, heating the applied layer to a temperav ture of about 150 C. to drive off the water and alcohol and cure the resin, superimposing a plurality of sheets of said coated magnetic material, and annealing the superimposed sheets at a temperature within the range of 900 C. to 1300 C. in an atmosphere of hydrogen gas to drive oil the resin whereby an electrical insulating coating of magnesium oxide and bentonite is left between the sheets.

4. The method of claim 3 wherein from 5% to 50% of the weight of the magnesium hydroxide is replaced by at least one finely divided inert refractory solid selected from the group consisting of metal oxides, metal silicates, and metal phosphates.

References Cited in the file of this patent UNITED STATES PATENTS 1,924,311 Frey Aug. 29, 1933 2,456,458 Somerville Dec, 14, 1948 2,465,284 Schmidt et a1. Mar. 22, 1949 2,600,473 Brockman June 17, 1952 2,623,920 Ford Dec. 30, 1952 2,680,285 Furnas June 8, 1954 2,743,172 Vaney Apr. 24, 1956 

1. IN THE METHOD OF FORMING MAGNETIC CORES, THE STEPS COMPRISING APPLYING TO MAGNETIC SHEET MATERIAL A LAYER OF AN AQUEOUS COMPOSITION CONSISTING ESSENTIALLY OF FROM 5% TO 15% BY WEIGHT OF AT LEAST ONE AMINOTRIAZINE-ALDEHYDE RESIN, FROM 5% TO 15% BY WEIGHT OF MAGNESIUM HYDROXIDE, FROM 0.1% TO 1% BY WEIGHT OF BENTONITE, FROM 30% TO 50% BY WEIGHT OF AT LEAST ONE LOW MOLECULAR WEIGHT ALIPHATIC MONOHYDRIC ALCOHOL, AND FROM 30% TO 50% BY WEIGHT OF WATER, HEATING THE APPLIED LAYER TO A TEMPERATURE OF ABOUT 150*C. TO DRIVE OFF THE WATER AND ALCOHOL AND CURE THE RESIN, SUPERIMPOSING A PLURALITY OF SHEETS OF SAID COATED MAGNETIC MATERIAL, AND HEATING THE SUPERIMPOSED SHEETS AT A TEMPERATURE 900*C. IN A REDUCING ATMOSPHERE TO DRIVE OFF THE RESIN WHEREBY AN ELECTRICAL INSULATING COATING OF MAGNESIUM OXIDE AND BENTONITE IS LEFT BETWEEN THE SHEETS. 